1. Field of the Invention
The invention relates to a molecular fluorine (F.sub.2) laser, and particularly to an F.sub.2 -laser having enhanced efficiency, line selection and line narrowing of the selected line.
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, to be followed by the next generation of ArF-excimer laser systems operating around 193 nm. Vacuum UV (VUV) is expected to use the F.sub.2 -laser operating around 157 nm.
The emission of the F.sub.2 -laser includes at least two characteristic lines around .lambda..sub.1 =157.629 nm and .lambda..sub.2 =157.523 nm. Each line has a natural linewidth of around 15 pm (0.015 nm). The intensity ratio between the two lines is I(.lambda..sub.1)/I(.lambda..sub.2).apprxeq.7. See V. N. Ishenko, S. A. Kochubel, and A. M. Razher, Sov. Journ. QE-16, 5 (1986). FIG. 1 illustrates the two above-described closely-spaced peaks of the F.sub.2 -laser spontaneous emission spectrum.
Integrated circuit device technology has entered the submicron regime, thus necessitating very fine photolithographic techniques. Line narrowing and tuning is required in KrF- and ArF-excimer laser systems due to the breadth of their natural emission spectra (&gt;100 pm). Narrowing of the linewidth is achieved most commonly through the use of a wavelength selector consisting of one or more prisms and a diffraction grating (Littrow configuration). However, for an F.sub.2 -laser operating at a wavelength of approximately 157 nm, use of a reflective diffraction grating is unsatisfactory due to its low reflectivity and high oscillation threshold at this wavelength. The tunability of the F.sub.2 -laser has been demonstrated using a prism inside the laser resonator. See M. Kakehata, E. Hashimoto, F. Kannari, M. Obara, U. Keio Proc. of CLEO-90, 106 (1990).
F.sub.2 -lasers are also characterized by relatively high intracavity losses, due to absorption and scattering in gases and all optical elements, particularly in oxygen and water vapor which absorb strongly around 157 nm. The short wavelength (157 nm) is responsible for the high absorption and scattering losses of the F.sub.2 -laser, whereas the KrF-excimer laser operating at 248 nm does not experience such losses. Therefore, the advisability of taking steps to optimize resonator efficiency is recognized in the present invention. In addition, output beam characteristics are more sensitive to temperature induced variations effecting the production of smaller structures lithographically at 157 nm, than those for longer wavelength lithography such as at 248 nm.